The present invention is related to a purification (removal and/or recovery) method and plant of metal(s) from waste water (aqueous media) by using a moving bed reactor inoculated with metal biosorbing and/or bioprecipitating and/or biotransforming bacteria.
The present invention is also related to a method to maintain a live bacterial population in a continuously operating sand filter, as wall as to a method whereby the metal(s) can be recovered from the produced metal laden biomass.
Metal containing waste waters are obtained from metal recycling or processing companies (non-ferrous) as well as surface treatment companies (galvanisation).
Surface treatment companies recycle 70-80% of the metal containing waste water. The remaining 30-20% must be treated and the metal removal is quite difficult due to the complexing properties of some organic compounds, causing high discharge costs.
Due to the strengthening of environmental standards, problems arise with the removal of trace concentrations (few ppm""s) possibly being complexed or added with organic contaminants.
These environmental problems urge for a simple system and regeneration technology which advantageously reduces sludge volumes for either metal recycling or final disposal.
The following table 1 give an estimation of the minimum amount of the metals Cu, Pb and Zn entering in the European surface waters if the present German standards were used by the European Union.
Normally volumes to be treated are in the order of magnitude between 25 and 200 m3/h for mining and metal processing plants and between 4 and 30 m3/h for surface treatment plants.
Physico-chemical methods like precipitation and electrolytical metal removal are difficult to apply or too costly for use at the given low concentrations. Ion exchange techniques are often inhibited by the presence of organic contaminants and is a relatively costly technique.
Different xe2x80x9cclassicalxe2x80x9d biological methods for cleaning heavy metals containing water have been proposed, like the use of helophytes as described in the US Patent U.S. Pat. No. 4,793,929.
Biosorption and bioprecipitation of metals have been described extensively (e.g. U.S. Pat. No. 5,055,402 and U.S. Pat. No. 5,279,745). In the biosorption technique, metals are bonded to functional groups on the cell surface polymers of the biomass. The bioprecipitation technique implies the crystallisation of heavy metals at the cellular surface. These processes can be performed with dead and/or structurally modified micro-organisms as described in the US Patent U.S. Pat. No. 4,021,368.
The nature of biosorption and bioprecipitation methods implies that the regeneration of the biomass is necessary.
The European Patent Application EP-0333218 describes a process for the removal of transition metals ions from aqueous solutions, which comprises the step of growing water fern Azolla in such solution until a substantial percentage of the initial metal ions content is taken up by the growing fern. About 80% of the water content of the fern can be mechanically removed and will contain no more than 1 to 1.5% of the accumulated metal content. The remainder of the metal remains bound in the biomass and can be recovered.
The International Patent Application WO95/23767 is related to a method and apparatus for processing water contaminated by organic compounds in a biologically activated and continuously operating granular filter bed. However, in said apparatus and method, the medium is not washed. Therefore, no separation of washwater containing biomass is performed. In addition, the turbulent transport of the filter medium from bottom up to the top surface of the filter bed reduces the bacterial stock in such a substantial way that the reactivity of the filter bed is noticeably deteriorated. This can be prevented by providing several transport units which do not cause a too important loss of the bacterial stock and thereby do not affect the reactivity of the filter bed. These several transport units are gas lifts as described in the International Patent Application WO95/00447.
The International Patent Application WO95/00447 describes an air-lift used to transport vertically a suspension constituted of particles of varying sizes, which will prevent the micro-organism strains from being decimated to an extend which will appreciably affect the effectiveness of the water purification process. This gas lift consists of the use of two or more transport tubes in parallel. The quantity of material transported in the individual transport tubes will be limited to a value at which reactivity is not impaired appreciably by excessive wearing away of the bacteria strains.
The International Patent Application WO81/022308 is related to a process and apparatus for converting organic materials in a micro-biological process under gas generation. In said method and apparatus, the conversion takes place in a continuous aqueous phase where a substrate for the micro-organisms is supplied to a moving bed of inorganic granular materials. However, the above-described method and apparatus are only related to the conversion of organic materials by micro-organisms, and these documents do not teach that it is possible to convert these techniques to a biological treatment of metals containing waste waters.
The U.S. Pat. No. 4,826,602 describes a process for reducing the concentration of ionic species of heavy metals in an aqueous waste solution. Said process comprises the step of contacting the waste solution with a culture of Pseudomonas maltophilia (ATCC 53510) in the presence of an amount of nutrient medium sufficient to satisfy the nutritional requirements of said bacteria. Said document also discloses the use of said technique in a xe2x80x9ccontinuous processxe2x80x9d which will not solve the above mentioned drawbacks when one applies said process in a continuous way. Any reactor used for such a purpose will need regeneration, and since no solution is provided for a continuous regeneration of the reactor while the amount of micro-organisms remains stable, said process can not be considered as a fully continuous process.
The U.S. Pat. No. 4,519,912 is related to a process for reducing (among others) the concentration of heavy metals species in an aqueous solution. The heavy metals of said solution are converted from soluble elements to insoluble elements which can be retained on a porous matrix being a support of anaerobic bacteria. Said heavy metals are then after recovered from the matrix.
More specifically, it is difficult to obtain by the processes of the state of the art a truly continuous process in the treatment of heavy metals containing waste waters. Indeed, said processes of the state of the art are mainly xe2x80x9csemi-continuousxe2x80x9d, which means that the amount of living micro-organisms is not maintained constant during the whole process and that a reactivation of the reactors with micro-organisms is necessary.
Therefore, the processes and apparatus of the state of the art need the use of several reactors working in parallel for the treatment of said metals containing waste waters.
As already mentioned biomass can be used for the removal of metals from water by biosorption (binding of metals to functional groups on the cell surface polymers) and bioprecipitation (crystallisation of metals at the cellular surface) processes. Several industrial processes based on said techniques were developed under the names BioFix beads(copyright), AlgaSorb(copyright), AMT-Bioclaim(copyright) systems. Testing of such immobilised biomass systems revealed several problems. The metal removal efficiency increases with increased concentrations of biomass. This indicates a need for large amounts of biomass. After saturation of the biomass, a regeneration with weak acids is necessary. The regeneration time increases with the biosorption cycles. Also, a complete regeneration is not possible after some cycles, which will therefore result in a decreased efficiency. In addition, either some metals cannot be removed, or some biomass is destroyed.
The present invention aims to provide a purification (removal or recovery) method and plant of metals containing aqueous media which do not present the drawbacks of the state of the art.
A main aim of the invention is to provide such a continuous method and plant (working in a continuous way) and which do not need a reactivation of the reactor with micro-organisms during the purification process.
Another aim of the invention is to provide a process and plant which do not need the parallel use of several reactor vessels.
A further aim of the invention is to provide such a method and plant wherein few or nor waste is generated, which do not require the conception of large amounts of chemicals for the purification, which are simple, easy to use and not expensive.
The present invention concerns a continuous purification (removal or recovery) method of metal(s) containing water wherein the water flows through a moving bed reactor which has been inoculated with viable micro-organisms, wherein a part of the micro-organism biomass, laden with said metals, is evacuated and wherein the biomass loss is maintained essentially equal to the biomass growth.
In the method according to the present invention, the supporting material of the moving bed reactor can be recirculated. The recirculation of the supporting material is preferably modified according to the micro-organism biomass growth.
According to the invention, said moving bed reactor is preferably a moving bed granular filter, and advantageously an AstraSand filter, as described in e.g. EP-A-0 730 895 and EP-A-0 590 705.
In said process, the natural biomass growth is regulated with means comprising feed for the micro-organism biomass and having one or more constituents selected from the group consisting of nitrogen source, carbon source, phosphate source, calcium source, sodium source, . . .
Advantageously, the purification method of metal(s) containing water according to the invention is obtained by biosorption or precipitation of the metals which will adsorb in a biofilm on the supporting granular medium (e.g. sand).
According to a preferred embodiment of the present invention, the micro-organisms are strains selected from the group consisting of Alcaligenes eutrophus CH34, Alcaligenes eutrophus ER121, Pseudomonas mendocina AS302, Citrobacter sp. N14, Arthrobacter sp. BP7/26 and BP7/15, Methylobacillus MB127 or a mixture thereof and already described in the literature.
According to another embodiment of the present invention, the method further comprises a biotransformation step.
The present invention is also related to the plant (or apparatus) for the continuous purification of metal(s) containing water based upon the method according to the invention.